1. Field of the Invention
The present invention relates to styrenic block copolymers and a process for preparing same. The present invention particularly relates to coupled styrenic block copolymers and a process for preparing same.
2. Background of the Art
The preparation of block copolymers is well known. In a synthetic method an initiator compound is used to start the polymerization of one monomer. The reaction is allowed to proceed until all of the monomer is consumed resulting in a living homopolymer. To this living homopolymer is added a second monomer that is chemically different from the first. The living end of the first polymer serves as the site for continued polymerization, thereby incorporating the second monomer as a distinct block into the linear polymer. The block polymer so grown is living until terminated.
Termination converts the living end of the block copolymer into a non-propagating species, thereby rendering the polymer unreactive toward additional monomers. A polymer so terminated is commonly referred to as a diblock copolymer. Alternately, the living block copolymers can be reacted with multifunctional condensing agents commonly referred to as coupling agents. Coupling of the living ends results in radial polymer having at least two branches, the branches being commonly referred to in the art as “arms”.
While the resultant block copolymers can have at least two arms, for certain applications, they preferably have three, four, or even more arms. For example, bitumen, also commonly referred to as asphalt, is a cost-effective thermoplastic material used in a wide variety of applications. However, especially in the more sophisticated applications, the performance properties of conventional bitumen have the disadvantage of being very sensitive to temperature. Often bitumen is too hard and brittle at low temperatures and too soft in hot weather conditions.
The incorporation of block copolymers having three and four or more arms into bitumen can significantly enhance the performance of the bitumen in road, runway, racetrack, and roofing applications, both by improving its life expectancy and by reducing maintenance requirements. Addition of such a block copolymer into bitumen introduces significantly superior properties to the modified bitumen. For example, such a modified bitumen can be much more elastic at both low and high temperatures.
While the advantages of using such block copolymers in bitumen are well known, the use of such block copolymers is not trouble free. For example, in the coupling process, the number of arms produced is not limited to three or four but can go on to include five or more. For purposes of the present invention, the terms “five or more” and “greater than or equal to five” are represented by the term 5+. When the number of arms to the block copolymer exceeds four, (or=5+) then the advantages of including the block copolymer in bitumen to improve its physical and performance properties can begin to be outweighed by resulting processing difficulties. For example, a five arm polymer produces a substantially greater increase in bitumen viscosity than a three arm polymer.
One object of the present invention is to provide a method for producing block copolymers having increased three and four arm contents and reduced five and higher arm contents. A further object is to provide a composition that can be used in processes such a bitumen modification to improve the physical properties of modified bitumen without causing significant processing difficulties.